The invention relates to a sewing or embroidery machine with a gripper which can be driven by a drive motor and with a lower thread bobbin rotatably mounted in the gripper for receiving a lower thread supply, with a hollow cylindrical arbor and annular flanges set on the arbor ends, of which at least one has perforations therethrough, or is provided with non-reflecting markings. More particulary, the invention relates to such sewing or embroidery machines which further include a light source and two photosensors for receiving the light rays emitted from the light source at two spaced-apart places, and a calculating and control unit for processing the signals supplied by the photosensors.
When sewing or embroidering with a sewing machine, it is known that two threads, the upper thread and the lower thread, are looped together. The upper thread, also termed the needle thread, is supplied from a spool whose size is substantially freely selectable, on or near the sewing machine. The lower thread is wound on a bobbin which is inserted within the rotatably mounted and drivable hook of the sewing machine and is freely rotatable there. The maximum size of the lower thread bobbin is thus determined by the maximum size of the hook within which it is situated. The amount of lower thread, or the lower thread supply, is in this case very much smaller than the upper thread supply located on the externally arranged spool, and moreover the lower thread bobbin is not visible from outside during sewing, since it is situated within the hook housing, which in turn is within the sewing machine. For this reason, monitoring of the current lower thread supply, the lower thread takeoff and the thread end during the embroidery or sewing process is difficult. Matters are complicated when the operator winds another thread onto an already partially filled lower thread bobbin. This other thread is not connected to that already present on the bobbin. No further sewing can be performed after this outer thread supply has been used up, although, for example, known sensors show that the bobbin core is still 50% or 70% full.
Measuring devices are already known in the prior art which seek to determine the end and/or the residual amount of the lower thread on the lower thread bobbin and which stop the sewing machine before the end of the lower thread is drawn through the sewn goods and before stitches are sewn which are consequently not held by a lower thread on the underside of the sewn goods.
From WO 82/04447, a device is known for detecting the thread end on a lower thread bobbin of a sewing machine. A light source emits a light ray substantially radially from outside through an opening in the hook housing in the direction of the rotation axis of the hook and the lower thread bobbin placed therein. Two photosensors are arranged spaced apart and tangentially offset from the light source and likewise directed substantially toward the rotation axis of the hook and of the lower thread bobbin, and are situated over a suitable aperture in the gripper housing. Flat places are formed on the core of the lower thread bobbin, and their surface is polished so that light rays directed from the light source toward the core of the lower thread bobbin are reflected at the flat places and the reflected rays can be successively received by the two photosensors as the empty bobbin rotates. As long as a thread supply is present on the bobbin, no reflection of light rays takes place. This is interpreted by the machine control to the effect that the sewing process can continue, because thread is still available. As soon as the thread is used as far as a single thread layer, and the light rays directed from the light source onto the bobbin can be reflected at the core or at the flat places and received by the photosensors, the machine control then detects the immediately approaching thread end. At the same time, it detects in which direction of rotation the lower thread bobbin is driven by the thread being taken off, in that the reflected rays reach the two photosensors, which are arranged one behind the other, in a corresponding sequence. By means of an annular coupling inserted between the lower thread bobbin and the hook housing, the lower thread bobbin is driven by the hook in the opposite direction as soon as the thread end has left the bobbin and therefore can no longer drive it. The light rays now reach the photosensors in the reverse sequence, and this is then detected as the yarn end and the machine is stopped. A residual length of lower thread thereby remains on the sewn goods. This device indeed makes it possible to detect the thread end, but for this purpose specially made lower thread bobbins with flat places are necessary. These are not commercially obtainable. Furthermore, the hook housing has to be provided with corresponding openings, in order to allow the light rays to enter the packing space and leave it again. A further disadvantage is that both the light source and the two photosensors are exposed to a lot of fluff accumulation and can hardly be cleaned by the seamstress. The thread end cannot be detected on overwound bobbins.
From DE-A 3046260, another method and a device for automatic sewing control on sewing machines have become known, in which the thread use of the sewing thread or respectively the lower thread of the sewn seam is monitored when each single stitch is sewn and is compared with an adjustable minimum value. Such expensive length measuring devices cannot be used in household sewing machines due to space requirements, and they are therefore little used, since in household sewing machinesxe2x80x94in contrast to industrial sewing machinesxe2x80x94the lower thread supply present on the lower thread bobbin is not exactly known. A length measurement is consequently of no use for detecting the thread end.
The object of the present invention is to provide a device with which the embroidery or sewing machine can be stopped as soon as the end of the lower thread falls below a predeterminable minimum length.
This object is attained by a sewing or embroidery machine a hook which can be driven by a drive motor and with a lower thread bobbin rotatably mounted in the hook for receiving a lower thread supply, with a hollow cylindrical arbor and annular flanges set on the arbor ends, of which at least one has perforations therethrough, or is provided with non-reflecting markings. A light source and two photosensors for receiving the light rays emitted from the light source at two spaced-apart locations are provided, as well as a calculating and control unit for processing the signals supplied by the photosensors. The light source and the two photosensors are arranged axially spaced in front of the flange with the perforations or markings, and the light rays are directed at an acute angle onto the surface of the flange situated in front. Advantageous embodiments of the invention are recited in the dependent claims.
With the sewing or embroidery machine according to the invention, the thread end of the last wound lower thread can be detected, independently of whether one or more threads are wound one on top of another on the lower thread bobbin, and the machine can be stopped in good time before the thread end leaves the stitch plate. Monitoring of a thread break or of the thread end can reliably take place independently of the make of the lower thread bobbin body.